Current ignitions typically use the more complex digitally processed or microprocessor REV limiting systems with two separate sets of code switches to allow user selectable REV limit and the number of engine cylinders, such as 4, 6 or 8 cylinders. Prior to the newer microprocessor based limiter, the more complex and expensive hard wired digital limiters replaced analog circuitry due to their significantly improved temperature stability. In contrast, the more recent microprocessor REV limiter replaces much of the hard wired digital circuitry, but additionally requires software to function.
A microprocessor REV limiter requires circuitry such as input trigger, output stages, and a stable clock oscillator for the processor chip to function and interface with the outside world. This forms the processor's basic hardware circuit which is internally controlled by software, i.e. a written set of programmed instructions that directs the entire operation of the processor. The software program is a set of detailed operating instruction on how the processor will function for sensing input ignition trigger signals, comparing that input trigger frequency to the REV limit selected by the user switches, accounting for the number of cylinders as selected by the cylinder switches, and how the output will occur when REV limit has been reached. Once the program has been written, debugged and proven reliable, it must then be download into each microprocessor integrated circuit (IC) chip for it to function as a REV limiter. This new approach accurately compares the elapsed time between ignition trigger pulses to that of a high frequency temperature stable crystal oscillator, also known as the microprocessor clock which steps the processor through each programmed instruction. The crystal oscillator frequency, typically in the mid megaHertz (mHz) range, is scaled down by division to an appropriate frequency as dictated by the particular application. Unfortunately, this very accurate approach to controlling RPM is complex and costly for such a simple task. Other techniques using analog circuitry are simpler but suffer from temperature instability. As engine ignition control electronics is normally subjected to wide temperature variations when operating in the proximity of the enclosed area of a hot engine, the thermal instability of the analog circuit limits its use for such applications.
However, by careful design consideration of the specific elements of analog circuitry that produces the thermal instability, the negative temperature coefficient of a semiconductor junction can effectively be canceled to produce a low cost, simple, thermally stable circuit that is well suited for ignition RPM control (REV limiting). Like its expensive microprocessor counter part, the analog RC timing circuit approach can easily incorporate all of the features for external user selectable RPM and the selection of the number of cylinders.
It is therefore an object of the present invention to limit RPM through an analog system, or substantially analog system with minor digital adjuncts, overcoming the disadvantages of the state-of-the-art digital and microprocessor based systems outlined above.